Accumulation means for reduction of sputter in gas-filled, lyman-alpha source tube



March 19 1968 G. J. BERGEN ET AL 7 3,374,383

ACCUMULATION MEANS FOR REDUCTION OF SPUTTER IN GAS-FILLED, LYMAN-ALPHA SOURCE TUBE Filed April 28, 1966 (f 7 u L 9 \k /L I If 4 l2 l 0.] 0.2 0.4 L0 2 4 IO 20 4O pd (mm. Hg.) x (CENTIMETERS) M INVENTORS' GEORGE J. BERGEN ARNOLD H. SINGER BY m ATTORNEY United States Patent Office 3,374,383 ACCUMULATION MEANS FOR REDUCTION OF SPUTTER IN GAS FILLED, LYMAN ALPHA SOURCE TUBE George J. Bergen, Oxon Hill, Md., and Arnold H. Singer, Washington, D.C., assignors to the United States of America as represented by the Secretary of the Navy Filed Apr. 28, 1966, Ser. No. 546,491 Claims. (Cl. 313-207) ABSTRACT OF THE DISCLOSURE A miniature gas filled cold cathode Lyman-Alpha source tube including means to prevent deleterious effects due tosputtering while providing a tube having stable operating characteristics throughout a long life span.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention relates to an electron conducting tube and more particularly to a gas-filled cold cathode Lyman-Alpha radiation source tube wherein the effects of sputtering, as well as clean-up, normally present during the operation of the electron tube are minimized providing stable tube characteristics and long life.

Electron conducting tubes are subjected to at least two phenomena during their normal course of operation including sputtering and clean-up. The instant invention is primarily concerned with the effects of sputtering, although clean-up is a prime factor in the design of this tube. Since both of these phenomena are natural occurrences in the normal operation of the tube, a particular tube must be designed in view of these phenomena, taking into consideration other limiting factors caused by the environmental conditions in which the tube is to be used, for example, available voltage supply, temperature etc.

Gas-filled cold cathode conducting tubes of the Lyman- Alpha radiation type, are frequently used as radiation sources. After continued operation of the prior art tubes the effects of sputtering will cause the lithium fluoride window to be covered with surface material so as to substantially impede transmission of radiation of energy through it, rendering the tube unsatisfactory for use as a standard of calibration for other radiation sources and detectors of this same initial wave length. In fact, such prior art tubes could not be satisfactorily used as radiation sources in connection with sensitive electronic equipment for any given time because of the effects of the sputter material on the lithium fluoride window, A further limitation on the useful life of this type of electron tube is caused by clean-up which acts to remove substantial amounts of hydrogen from the initial supply placed in the sealed tube thus substantially altering its operating characteristics.

The general purpose of this invention is to provide a gas-filled cold cathode Lyman-Alpha source tube which embraces all the advantages of similarly employed source tubes and possesses none of the aforedescribed disadvantages. To obtain this, the present invention contemplates a novel gas-filled cold cathode Lyman-Alpha source tube of generally cylindrical shape being capable of transmitting radiation of approximately 1215 A., comprising two elongated conductive electrodes extending through one end of the tube along a generally longitudinal axis thereof and having a means within the container adjacent the end of the wires whereby the contaminant effects caused by sputtering in the normal operation of the tube is prevented Patented Mar. 19, 1968 permitting the tube to contain sufficient initial amount of hydrogen thus avoiding the effects of clean-up on the operating characteristics of the tube during its long life span.

An object of the invention is the provision of an electron conducting tube having relatively long life with stable operating characteristics throughout its life.

Another object of the invention is the provision of an electron conducting tube constructed to reduce the effects of sputtering and clean-up on its operating characteristics during its life.

Another object of this invention is a provision of a gas-filled Lyman-Alpha source tube having long life and relatively stable operating characteristics throughout so as to render it useable as a standard for calibration of similar conducting devices.

A further object of this invention is a provision of electron tube suitable for use as a Lyman-Alpa radiation source tube being uniquely constructed so as to substantially eliminate the effect of sputtering on the walls of the tube.

Yet another object of the present invention is a provision of a gaseous discharge tube useable as a source of radiation having relative stable operating characteristics during its long life.

A still further object of the present invention is a provision of a gas-filled cold cathode Lyman-Alpha radiation source tube having a long life of useful operation, and requiring minimum starting and substaining operating voltage providing low background and circuit noise during its operation, yet maintaining the tubes inner surface free from sputtered materials, the tubes operating characteristics being viitually unaffected by the effects of hydrogen cleanup Other objects and features of the invention will become apparent to those skilled in the art as the disclosure is made in the following description of a preferred embodiment of the invention as illustrated in the accompanying sheet of drawing in which:

FIG. 1 shows a cross-sectional view of a preferred embodiment of the invention;

FIG. 2 shows a graphical interpretation of Paschens law relating to voltage breakdown in gas discharged tubes for hydrogen.

Referring to FIG. 1 there is shown a glass enclosure 5 of generally cylindrical shape having electrode 6 and 7 protruding through one of its ends. At its opposite end is a lithium fluoride window 9 bonded to enclosure 5 with a suitable sealing means such as silver chloride, by methods well known in the glass forming art to provide a seal. An annular lip 8 formed by part of the enclosure 5 acts as an attaching means to enable a compatible detector (not shown) to be attached thereto for receiving energy transmitted through lithium fluoride window 9 when the tube is operating. Also formed by part of the enclosure 5 is a capillary tube 13 extending along the longitudinal axis of the enclosure 5 having an end adjacent, but not touching, lithium fluoride Window 9. Two holes 14 of diameter slightly greater than the diameter of electrodes 6 and 7 are provided at one end of capillary tube 13 for receiving the respective electrodes. Although various sizes of tubes may be used in a particular environment, this tube illustrated is miniaturized to have a length from lip S to the opposite end of the tube of two inches, and a diameter of 22 millimeters. Two componential dimensions are considered limiting in the operation of this particular device, namely spark-gap distance 12, and the distance 11 from the end of the capillary tube to the electrode gap. These dimensions were found to be suitable at 0.062 inch and 0.250 inch, respectively. The reasons for selecting these particular dimensions will be further explained in connection with the Theory and operation of the invention discussed hereinafter.

FIG. 2 is an exponential graph havin an abscissa representative of the product of tube pressure and sparkgap distance in millimeters of mercury-centimeters, and an ordinate representative of breakdown volts, respectively. This particular graphic representation of Paschens law is of hydrogen at a temperature of centigrade.

Theory and operation of the invention In the design of electron tubes many factors are considered influential in the structural design of a given tube. This invention is concerned with three of such considerations (excluding external environmental factors) which for purposes of this discussion will be called phenomena. They are sputtering, clean-up, and Paschens law, the latter two of which are interrelated with each other. While the above three phenomena are major considerations in the design of this electron tube, the prime external limiting factor is of course, the environment in which this particular tube is used, including such things as available voltage supply, temperature, etc. The particular environment which this tube is to be used has a low available supply voltage and requires stable operating characteristics with a long life of operation, absence of noise, together with miniaturization of physical size. With these limiting factors in mind the discussion of the three phenomena mentioned above will be more readily understood.

The first of these phenomena, sputtering, is caused by the bombardment of charged surfaces (electrodes) by oppositely charged ionic species so that the surface material of the charged surface is ejected into the gaseous phase in small atomic size particles. Because of their minuteness in size these particles become a component of the gaseous phase itself. In fact, the particles may themselves be considered a gas for purposes of this discussion, resulting in Brownian Motion. This random motion of the particles caused by Brownian Motion will result in particles colliding with themselves, and more importantly, colliding with internal surfaces in the tube where they remain indefinitely. Obviously after some length of time a sutficient number of these particles will accumulate on a given surface in the electron tube causing a metallic coating to form thereon. In the instant device, where the tube is used as a radiation source having its energy directly transmitted through the transparent lithium fluoride window 9, this metallic coating will cause the amount of energy transmitted to decrease as the tube continues to operate resulting in a thickened coating. By placing an accumulating means such as capillary 13, adjacent the area of the electrodes where the sputtering occurs, the accumulations due to the effect of sputtering will occur on this deflecting means rather than the lithium fluoride window 9, thus tending to insure that the amount of radiation available from the tube is constant throughout its operative life.

The second of these phenomena, clean-up, refers to the removal of the initial hydrogen gas in the electron tube from its gaseous phase by any of several means including, chemical reaction of the gas with other atoms or molecules to form new molecular species, absorption of the gas into the inner walls of the tube, and adsorption of the gas on the inner surface of the tube. After a given length of time (dependent upon the amount of gas initially placed in the tube) has elapsed in the normal operation of the tube, the original supply of gas will be sufficiently cleaned-up resulting in a substantial-change in the operational characteristics of the tube. Obviously a larger supply of gas will provide a longer period of clcan-up. The amount of gas in an electron tube (assuming given volume of the electron tube using increased pressure to obtain a larger supply of gas in accordance with fundamental principles of fluid statics) has a direct affect on the breakdown and operating voltage requirements. This is best illustrated by referring to FIG. 2 showing voltage breakdown curve of hydrogen in accordance with Paschens law. In trying to maintain a minimum voltage breakdown throughout the life of the electron tube, while at the same time insuring an adequate supply of hydrogen in the tube, it becomes necessary to select a proper spark-gap distance, which is itself limited by the available power supply for voltage breakdown. Thus, with a minimum voltage supply available and the need for an adequate supply of hydrogen, it becomes readily apparent that an important consideration in the design of such an electron tube is the spark-gap distance, as well as the small volume available within the tube itself due to the need for miniaturization. With these factors optimumly selected, the device provides a miniaturized gas-filled cold cathode Lyman-Alpha radiation source tube having stable characteristics with a minimum of noise during its long life of operativeness.

Several dimensions of componental elements in the tube were found to be somewhat limiting in selectivity for optimum results. For example, the electrode gap, or spark-gap, was chosen to be 0.062 inch of a tolerance of or 0.020 inch. This proved to be optimum in view of voltage breakdown requirement in conjunction with the necessary amount of hydrogen gas needed in the tube for operation at a given temperature. The tube was found to operate properly with an initial supply of approximately 7-10 millimeters of mercury of hydrogen so as to be operable with a supply voltage of 300 volts.

A second limiting dimension was found to be the distance between the end of the capillary adjacent the lithium fluoride window to the edge of the holes in which the electrodes are inserted in the capillary, the distance being preferably 0.250 inch for best results. Two considerations influenced this choice. The first is that the distance must be long enough to ensure the random motion particles caused by Brownian Motion to contact this surface at least once before reaching the lithium fluoride window to prevent contact of the particles therewith. A second consideration, though somewhat less critical, is the need to avoid substantial reduction in the Lyman-Alpha output caused by readsorption, scattering, etc. which would result if the distance between the lithium fluoride window to the electrode gap was of too great a length.

Many modifications are contemplated within the scope of this device. For example the electrodes may be coated with an insulated material to avoid occurrence of a discharge at any position other than the electrode gap. Fur thermore the deflection means itself does not necessarily have to be a capillary tube and may, for example, take the form of two parallel plates of proper dimensions and location or any other similarly arranged means for accumulating the sputtered particles thereon.

It should be understood, of course, that the foregoing disclosure relates to only a preferred embodiment of the invention and that numerous modifications or alterations may be made therein without departing from the spirit and the scope of the invention as set forth in the appended claims.

What is claimed and desired to be secured by Letters Patent of the United States is: v

1. An electron conducting tube comprising:

a generally cylindrical, gas-filled container including a window for transmission of radiation therethrough; at least two conductive elongated wires extending through one end of said container along a generally longitudinal axis of said container and having ends 5 6 terminating adjacent said window whereby a gas a lithium fluoride window disposed across said discharge may occur between said ends; and opening and sealed to said container so as to accumulating means within said container adjacent the render said container gas tight.

internal ends of said Wires for preventing con- 4. The device as defined in claim 1, for transmitting taminant effects caused by sputtering when the tube 5 radiation with a wavelength of about 1215 A., wherein; is conducting from accumulating on the walls of the ends of said elongated wires within said container said container thereby insuring maximum passage of face each other with a distance of 2 mm. or less radiation through the window of said container. separating the ends thereof; and 2. The device as defined in claim 1, wherein; the container is filled with commercial dry hydrogen to said accumulating means is a cylindrical capillary tube '0 a pressure between approximately 7-12 mm. of Hg open at its end adjacent said Window in said conso as to permit the tube to operate with an energizatainer, said accumulating means being mounted withtion supply of between 300 to 400 volts. in said container so as to have its longitudinal axis 5. The device as defined in claim 2, wherein; along the longitudinal axis of said container, said the distance between the end of the capillary tube to capillary tube having further oppositely disposed 15 the electrode gap is approximately 0.250 inch. openings in its side area for receiving the inner ends of said wires so as to cause the accumulation of con- References Cited taminants caused by sputtering to be substantially UNITED STATES PATENTS confined t0 the inner 'Walls Of said capillary tube. 3. The device as defined 1n cla1rn 1, wherein 20 2,561,898 7/1951 willoughby said contalner has an opening in its end ad acent the 3,180,990 4/1965 Randall at a]. 313 204 X open end of said accumulation means, and further including; ROBERT SEGAL, Primary Examiner. 

